Drainage system with membrane cover and method for covering wastewater reservoir

ABSTRACT

A membrane cover is mounted at the surface of a wastewater reservoir. The membrane cover has a flexible buoyant impermeable membrane affixed to the perimeter walls of the reservoir and loosely laid over the wastewater level inside the reservoir. An array of weight lines is anchored to the perimeter walls and is loosely laid over the impermeable membrane. The array of weight lines defines a rectangular herringbone pattern. An array of drains is also provided. Each drain is disposed along one of the lateral weight lines. The membrane cover also comprises an array of troughs formed therein under the array of weight lines. A water ballast having the continuous herringbone pattern is contained within the troughs. Because of this water ballast, the membrane cover is kept taut and stable during a rainstorm and a surface thereof exposed to uplifting wind forces is minimized.

FIELD OF THE INVENTION

This invention pertains to drainage systems for flexible floating coverscovering large wastewater reservoirs, and particularly it relates to adrainage system for accumulating rainwater on a membrane cover in acontrolled manner and for draining excess rainwater into the wastewaterreservoir.

BACKGROUND OF THE INVENTION

Membrane covers are mounted over settling ponds, clarifiers and otherwastewater reservoirs to contain and collect fermentation gases of milleffluent for example. The maintenance of a membrane cover floating on alarge wastewater reservoir represents certain difficulties in that thecover is exposed to the elements and to the movement of the liquid underthe cover. For example, a slight accumulation of rainwater over a covertends to create puddles, mounds and relaxed segments at random locationson the cover. These puddles, mounds and especially the relaxed segmentscatch the wind and promote waves along the cover and into the liquidunder the cover. The movement of liquid under the cover causestangential stresses and constant movement in the membrane cover itself.These movements and stresses could cause fatigue, localized elongationand rupture of the cover. The formation of mounds, puddles and relaxedsegments on a membrane cover is amplified where the content of thecovered reservoir contains gases or is able to generate gases that tendto create gas pockets at the surface of the wastewater under themembrane cover.

Although the formation of rainwater puddles and mounds over a floatingcover is a natural phenomenon that will remain an inherentcharacteristic of a flexible membrane cover, it will be appreciated fromthe following disclosure that there are numerous advantages which can bederived from an accumulation of rainwater over a floating membrane coverwhen the accumulation and drainage of rainwater are effected in acontrolled manner.

It is believed that the prior art is short on suggestion with regards toa drainage system to advantageously control the accumulation ofrainwater on a membrane cover. Examples of the drainage systems of theprior art for floating covers are described in the following documents:

U.S. Pat. No. 2,531,898, issued on Nov. 28, 1950 to R.C. Ulm, disclosesa floating roof with a flexible deck and a central weight mounted on thedeck. The central weight causes the deck to curve downward such thatrainwater flows downward toward the central weight. Rainwater isevacuated through a drain at the centre of the weight and through a hoseextending from the drain.

U.S. Pat. No. 4,672,691, issued on Jun. 16, 1987 to DeGarie et al.discloses a flexible membrane cover having weight lines thereon topromote the accumulation of rainwater under the weight lines. Rainwateris evacuated by evaporation or by the use of sump pumps placed directlyon the membrane cover.

U.S. Pat. No. 5,946,743, issued on Sep. 7, 1999 to I. S. Hashmi,discloses a pool cover having a drain hole at the centre thereof. Aflexible conduit extending from the drain pipe carries rainwater outsidethe pool.

Although the drainage systems of the prior art deserve undeniable merit,there is no known prior art that discloses, teaches or suggests adrainage system to control the accumulation of rainwater on a membranecover such that the cover is less susceptible of being exposed todestructive stresses from wind-induced liquid movement under the cover.

SUMMARY OF THE INVENTION

In the present invention, there is provided a membrane cover and adrainage system therefor whereby rainwater is quickly accumulated duringthe early stages of a rainstorm to provide a water ballast on themembrane cover to keep the cover taut and to limit the formation ofrandomly spaced puddles, mounds and relaxed segments that can deform thecover and create destructive stresses in the cover structure.

In a first aspect of the present invention there is provided awastewater reservoir having a membrane cover mounted thereon. Themembrane cover has a flexible buoyant impermeable membrane affixed tothe perimeter walls of the reservoir and loosely laid over thewastewater level inside the reservoir. An array of weight lines isanchored to the perimeter walls of the reservoir and is loosely laidover the impermeable membrane. The array of weight lines comprises alongitudinal weight line and a plurality of spaced-apart lateral weightlines extending perpendicularly from the longitudinal weight line onboth sides of the longitudinal weight line. An array of drains is alsoprovided. Each drain is disposed along one of the lateral weight linesand extends through the impermeable membrane. The membrane coveraccording to the present invention also comprises an array of troughsformed therein under the array of weight lines. The troughs areconnected to each other. A water ballast is contained within theconnected troughs and a level of water in this water ballast iscontrolled by the drains. The water ballast defines a herringbonepattern extending over a major portion of the cover.

A first advantage of the membrane cover having the water ballast thereonis that is the cover is kept taut and stable during a rainstorm and asurface thereof exposed to uplifting wind forces is minimized.

In another aspect of the present invention, the membrane cover comprisesa flexible buoyant layer affixed to the perimeter walls of the reservoirand loosely laid over the wastewater level inside the reservoir. Theimpermeable membrane is also affixed to the perimeter walls and isloosely laid over the flexible buoyant layer. In this aspect of theinvention, the membrane cover has a substantial thickness. Each drainhas a straight pipe extending above the upper surface of the membranecover for accumulating a level of rainwater in each trough and formaintaining a water connection between the troughs. The drains arelocated in a one-quarter portion of the width of the cover nearest oneof the perimeter walls.

The water ballast contained in the troughs is advantageous for itsexpanse and shape. The water ballast covers a major area of the membranecover without having large cross dimension exposed to up-lifting windforces. Because of the drains, the water ballast remains present on theimpermeable membrane for extended period of time following a rainstorm.

In a further aspect of the present invention, the thickness of themembrane cover cooperates with the drains to maintain the level of thewater ballast higher than the level of wastewater inside the reservoir,for draining rainwater in excess of the effective level of the waterballast.

Another advantage of the structure of the membrane cover is that theflexible buoyant layer, the impermeable membrane and the array of weightlines constitute three separate layers that are loosely laid over eachother. These three separate layers are therefore free to slide upon eachother and flex to follow the movement of the wastewater inside thereservoir without generating any destructive tangential stress in themembrane cover.

In another feature of the present invention the membrane cover furtherhas an array of ridges formed thereon each of which being disposedbetween two of the lateral weight lines. These ridges are advantageousfor offering gas passages to evacuate off-gases that are often generatedby the wastewater inside the reservoir.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is illustrated in theaccompanying drawings, in which like numerals denote like partsthroughout the several views, and in which:

FIG. 1 is a partial perspective view of a membrane cover and drainagesystem therefor according to a preferred embodiment of the presentinvention installed over a wastewater reservoir;

FIG. 2 is a top view of the wastewater reservoir having the membranecover installed thereon;

FIG. 3 illustrates a cross-section view of one of the drains through themembrane cover, shown without any accumulation of rainwater over themembrane cover;

FIG. 4 illustrates a second cross-section view of a drain in themembrane cover, as seen along line 4—4 in FIG. 2 and showing anequilibrium volume of rainwater on the membrane cover;

FIG. 5 illustrates a third cross-section view of the drain, shown duringthe evacuation of excess rainwater from the membrane cover;

FIG. 6 shows an enlarged cross-section view of the drain having anoptional flap valve mounted therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While this invention is susceptible of embodiments in many differentforms, there is shown in the drawings and will be described in detailsherein one specific embodiment of the present invention, with theunderstanding that the present disclosure is to be considered as anexample of the principles of the invention and is not intended to limitthe invention to the embodiment illustrated and described.

A preferred embodiment of the membrane cover and the drainage systemtherefor according to the present invention is partly illustrated in theaccompanying drawings. These partial drawings are provided herein forclarity. These drawings are believed to be sufficient for illustratingthe concept and principles of the present invention. Numerous otherstructural details or variations may be included in a complete coverinstallation. However, these additional structural details andvariations are known to those skilled in the art. The membrane cover anddrainage system therefor according to the present invention is animprovement over the floating membrane cover described in U.S. Pat. No.4,672,691 of which the first named inventor is also the inventor of thepresent invention.

The membrane cover according to the preferred embodiment comprises aflexible water-impermeable membrane 20 which is anchored to theperimeter walls 22 of a wastewater reservoir. A flat bar 24 and a seriesof anchor bolts 26 are used for clamping the impermeable membrane 20 tothe top edge of the walls 22. The impermeable membrane 20 has sufficientsurface to cover the reservoir at its lowest operating level. Theflexible impermeable membrane 20 is preferably made of astretch-resistant nylon-based pliable sheet material.

The impermeable membrane 20 is loosely supported at the surface of thereservoir by a flexible buoyant layer 30. The flexible buoyant layer 30is preferably made of a semi-rigid, closed-cell, water-impermeableinsulating foam or other semi-rigid low density insulating foam such aspolyethylene foam for example. For convenience, the flexible buoyantlayer is hereinafter referred to as the foam layer 30.

The foam layer 30 is also loosely laid at the surface of the wastewaterinside the reservoir and is anchored to the perimeter walls 22 with theimpermeable membrane 20. The impermeable membrane 20 and the foam layer30 are not attached to each other except at the perimeter of thereservoir and at the drain holes, as will be explained later. Theimpermeable membrane 20 and the foam layer 30 are thereby free to slideupon each other to increase the flexibility of the membrane cover suchthat the membrane cover can follow the movements of the waves in thewastewater of the reservoir without generating any tangential stresstherein.

The membrane cover according to the preferred embodiment also comprisesa longitudinal weight line 32 and an array of lateral weight lines 34laid over the impermeable membrane 20. The longitudinal weight line 32extends along a longitudinal median of the membrane cover. The lateralweight lines 34 are equally spaced-apart and extend at right angle fromthe longitudinal weight line 32. The longitudinal and lateral weightlines 32, 34 jointly define a herringbone pattern of weight lines.

The longitudinal and lateral weight lines 32, 34 are made of a pluralityof pipe sections filled with sand or concrete for example. The pipesections in the longitudinal and lateral weight lines 32, 34 are linkedto each other by rope 36 or light cable, such that each weight line islongitudinally flexible to follow the movement of the membrane coverwith any wave action in the covered liquid. The outside end of eachweight line 32, 34 is anchored to the perimeter walls 22 of thereservoir by means of anchor cables 38 attached to anchor tabs 40mounted to some of the anchor bolts 26 above the membrane clamping flatbar 24. The rope 36 of a weight line 34 above a drain 42 is preferablyattached to the drain 42, to retain the weight line to that drain.Because the weight lines 32, 34 are relatively flexible, their movementsrelative to the impermeable membrane 20 do not apply any significantconcentrated shear stress on the impermeable membrane.

A first function of the longitudinal and lateral weight lines 32, 34 isto cause longitudinal and lateral depressions along the surface of thecover to accumulate rainwater on the membrane cover according to aspecific pattern. During a rainstorm, the accumulation of rainwaterforms a water ballast over the membrane cover to stabilize the cover andto prevent excessive wind-induced movement in the membrane cover. Themembrane cover is thereby kept taut and the surface of the impermeablemembrane 20 exposed to uplifting wind forces is reduced.

The preferred water ballast comprises a central segment 50 which extendsalong the longitudinal weight line 32 and a plurality of spaced-apartlateral segments 52 respectively extending under one of the lateralweight lines 34. The central segment 50 and lateral segments 52 of thewater ballast also define a herringbone pattern which is betterillustrated in FIG. 2. It has been found that a water ballast having theillustrated herringbone pattern is more effective than other patterns instabilizing a membrane cover in periods of high winds during or after arainstorm for example. The expanse of this pattern covers a majorportion of the cover without exposing large cross dimension to the windforces and to the formation of wave at the surface of the water ballast.

In order to maintain a stabilizing effect on the membrane cover, it isimportant that a water connection is maintained between the lateralsegments 52 and the central segment 50 of the water ballast. It is alsoimportant to maintain the integrity of the central segment and of eachlateral segment. If water connection is broken along one segment, themembrane surface at that location is relaxed from its preferredstretched mode. A gas pocket may form at that location under the coverand cause the formation of a mound. A mound or a relaxed segment in amembrane cover is known to catch the wind and cause a water connectionin another segment to break and form another mound, and so on.Therefore, a broken water connection can entrain the formation of anumber of randomly-spaced mounds, puddles and relaxed segments, and theassociated undesired movements of the wastewater under the membranecover.

As will be explained, the water connections between and along thelateral segments 52 and the central segment 50 are maintained by aseries of drains 42 through the impermeable membrane 20 and the foamlayer 30.

Still referring to FIG. 2, the drains 42 are preferably located alongthe lateral weight lines 34 at a distance ‘A’ of about one quarter ofthe full width ‘B’ of the reservoir, measured from each side of thereservoir. The structure and position of the drains 42 in the sideportions of the floating membrane cover are also advantageous formaintaining a water connection along the water ballast segments 50, 52especially in installations where scum tend to accumulate at the centreof the membrane cover.

The foam layer 30 causes the formation of ridges 54 between the lateralweight lines 34. These ridges 54 offer gas passages under the membranecover whereby any off-gases generated by the content of the reservoircan be evacuated along the ridges 54 and toward the perimeter walls 22of the reservoir, such as illustrated by arrows 56.

The combination of the foam layer 30 and the water ballast 50, 52 havingthe herringbone pattern are advantageous for creating the ridges 54while maintaining the entire membrane cover under tension. The formationof a ridge 54 between two lateral segments 52 of the water ballast isadvantageous for providing a means to evacuate off-gases from under themembrane cover without causing any relaxed segment in the cover. On theother hand, the presence of ridges 54 on the membrane cover helps toaccumulate rainwater in the water ballast and to confine the waterballast to the illustrated herringbone shape. The membrane cover istherefor held under tension by both the water ballast and the ridges, asboth the buoyancy of the foam layer at the ridges and the weight of thewater ballast between the ridges act in opposite directions. Because theridges are under tension, the wind forces on the ridges 54 have minimaleffect on the movements of the wastewater under the cover. Because ofthe relatively narrow segments 50, 52 of the water ballast, the windforces on the water ballast also have minimum effect on the formation ofwaves in the water ballast.

Referring now to FIGS. 3-5, the operation of the drains 42 will beexplained in details. Each drain 42 consists of a straight pipe 60having a flange 62 affixed to the impermeable membrane 20 and to thefoam layer 30. The straight pipe 60 extends under the foam layer 30 adistance ‘C’ which is more than the thickness of the scum layer that isexpected to form under the membrane cover. The dimension ‘C’ is definedfrom experience with similar covered liquids. Most commonly, thedimension ‘C’ is between about six inches and about twelve inches.

The straight pipe 60 extends above the impermeable membrane 20 adistance ‘D’ which consists of an equilibrium dimension ‘E’ and aprecautionary dimension ‘F’. In a large membrane cover having few toseveral acres in surface for example, the equilibrium dimension ‘E’ maybe between about one and about two inches. However, for greatercertainty, the equilibrium dimension ‘E’ is preferably defined accordingto the following method at every new cover installation.

a) On an installed cover, all the drain holes are cut out at thedimensions ‘A’ along the lateral weight lines 34.

b) The portions of the cover under the weight lines 32, 34, are allowedto sink below the level of wastewater inside the reservoir, to partlysubmerge the weight lines 32, 34 in backwater and to establish a waterconnection above the cover and along the weight lines 32, 34.

c) The weight of the weight lines 32, 34 may be increased or decreasedto clearly defme the water ballast 50, 52 having a continuousherringbone pattern as mentioned before.

d) When equilibrium has been reached and that the continuity of thewater ballast is clearly defined, the depth of the backwater above theimpermeable membrane 20 at one of the drain holes is measured. Thisdepth of backwater above the impermeable membrane at the equilibriumstate corresponds to the equilibrium dimension ‘E’.

e) The straight pipe 60 of each drain 42 is then trimmed to thecorresponding dimension ‘D’.

f) The drains 42 are affixed to the cover and the backwater is pumpedunder the cover.

During a rainstorm, the equilibrium dimension ‘E’ contributes to quicklyaccumulate a water ballast on the membrane cover in the early stages ofa rainstorm to stabilize the membrane cover during and after therainstorm. The equilibrium dimension ‘E’ constitutes an equilibriumstate where a water connection is reliably maintained between thecentral segment 50 and the lateral segments 52 of the water ballast.

Although the above method makes use of backwater to define anequilibrium dimension ‘E’, it will be appreciated that the equilibriumdimension can also be defined using rainwater during a rainstorm forexample or using freshwater from another source. In these latter cases,the step of pumping backwater under the cover can be eliminated as thewater ballast is preferably left on the membrane cover to slowlyevaporate.

In FIG. 3 there is illustrated a cross-section view of the membranecover in a dry state, that is when rainwater has evaporated from itsupper surface. In this state, the wastewater level 64 inside thereservoir is preferably slightly higher than the surface 66 of theimpermeable membrane immediately under the weight line 34.

The weight lines 32, 34 and the flexibility of the membrane cover causethe membrane cover to slope outwardly upward from each weight line 32,34 to form an array of troughs 70 each having sloping sides 72. Thetroughs 70 are connected to each other, and the array of troughs 70 alsohas the aforesaid herringbone pattern. The sloping sides 72 and theridges 54 mentioned before cooperate to accumulate rainwater in theconnected troughs 70 to create the preferred water ballast 50, 52.

The purpose of the precautionary dimension ‘F’ mentioned before is toprevent drainage of the water ballast from the equilibrium state, asshown in FIG. 4, which could be caused by waves at the water surface orby wind-induced movements in the membrane cover. For membrane covershaving one acre or more in size, the preferred dimension ‘F’ is aboutone-half inch.

As rainwater accumulates in the troughs 70, the level 74 of the waterballast 50, 52 above the impermeable membrane 20 tends to remain higherthan the wastewater level 64 under the membrane cover. It has been foundthat the difference ‘G’ between the two levels is similar to or slightlymore than the thickness ‘T’ of the membrane cover. It is believed thatthe difference ‘G’ between the two levels can be slightly larger thanthe dimension ‘T’ due to the additional buoyancy provided by theoff-gases, excess scum or other floating particles accumulating at theridges 54. In the preferred embodiment, the thickness ‘T’ of themembrane cover is between about one-half inch to about three-quarter ofan inch. Therefore, as rainwater accumulates over the membrane cover,the excess water flows over the drain pipe 60 and is efficiently draineddown into the covered liquid, as illustrated in FIG. 5.

Referring now to FIG. 6, the flange 62 of a drain 42 is affixed to theimpermeable membrane 20 and to the foam layer 30 to form an impermeablejoint 80 with the membrane cover. In the preferred embodiment, thisimpermeable joint 80 is a bolted assembly of gaskets and annular plates.

In applications where the covered wastewater does not contain floatinggreases or similar scum, a flap valve 82 may be mounted inside thestraight pipe 60. The flap valve 82 is advantageous for preventing aback flow of wastewater through the drain 42 when a person walks nearthe drain 42, for inspecting the membrane cover for example. Thestraight pipe 60 also has a threaded portion 84 on its upper end forreceiving a pipe coupling and extension (not shown) or a pipe cap (notshown) for adjusting the equilibrium dimension mentioned before or forperforming repair work on the cover for example.

It will also be appreciated that the foam layer 30 can be omitted fromthe structure of the membrane cover in installations where grease orother buoyant scum accumulates at the surface of the wastewater.

While one embodiment of the present invention has been illustrated inthe accompanying drawings and described hereinabove, it will beappreciated by those skilled in the art that various modifications,alternate constructions and equivalents may be employed withoutdeparting from the true spirit and scope of the invention. Therefore,the above description and the illustrations should not be construed aslimiting the scope of the invention which is defined by the appendedclaims.

I claim:
 1. In combination, a wastewater reservoir and a membrane covermounted over said wastewater reservoir; said wastewater reservoir havinga wastewater level and perimeter walls, and said membrane covercomprising: a flexible buoyant impermeable membrane affixed to saidperimeter walls and loosely laid over said wastewater level; an array ofweight lines anchored to said perimeter walls and loosely laid over saidimpermeable membrane, said weight lines defining a rectangularherringbone pattern comprising a longitudinal weight line and aplurality of spaced-apart lateral weight lines extending perpendicularlyfrom said longitudinal weight line on both sides of said longitudinalweight line; an array of drains each of which being disposed along atleast one of said lateral weight lines and extending into the wastewaterthrough said impermeable membrane; an array of troughs formed thereinunder said array of weight lines; said troughs being connected to eachother; and a water ballast contained in said troughs; such that saidmembrane cover is kept taut and stable during a rainstorm, and a surfacethereof exposed to uplifting wind forces is minimized.
 2. Incombination, a wastewater reservoir, a membrane cover mounted over saidwastewater reservoir; said wastewater reservoir having a wastewaterlevel, a length, a width and perimeter walls defining said length andwidth, and said membrane cover having: a longitudinal median relative tosaid length and width; a flexible buoyant layer affixed to saidperimeter walls and loosely laid over said wastewater level; animpermeable membrane affixed to said perimeter walls and loosely laidover said flexible buoyant layer; an array of weight lines anchored tosaid perimeter walls and loosely laid over said impermeable membrane,said weight lines comprising a longitudinal weight line extending alongsaid longitudinal median, and a plurality of spaced-apart lateral weightlines extending perpendicularly from said longitudinal weight line onboth sides of said longitudinal weight line; said array of weight linesdefining a rectangular herringbone pattern; an array of drains, each ofwhich being disposed along at least one of said lateral weight linesnear said perimeter walls and extending into the wastewater through saidflexible buoyant layer and said impermeable membrane; said membranecover also having an array of troughs formed therein under said array ofweight lines; said troughs being connected to each other; and a waterballast contained in said troughs; such that said membrane cover is kepttaut and stable and a surface thereof exposed to uplifting wind forcesis minimized during a rainstorm.
 3. The combination as claimed in claim2, wherein said water ballast has said herringbone pattern.
 4. Thecombination as claimed in claim 2, wherein each of said troughs hassloping sides.
 5. The combination as claimed in claim 4, wherein saidmembrane cover further comprises an array of ridges formed therein eachof which being disposed between two of said lateral weight lines.
 6. Thecombination as claimed in claim 2, wherein each of said drains comprisesa straight pipe extending above a surface of said impermeable membrane.7. The combination as claimed in claim 2, wherein said drains arelocated in a one-quarter portion of said width nearest one of saidperimeter walls.
 8. The combination as claimed in claim 2, wherein saidimpermeable membrane is made of a stretch resistant nylon-based pliablesheet material.
 9. The combination as claimed in claim 2, wherein saidflexible buoyant layer is made of a water-impermeable insulating foam.10. The combination as claimed in claim 2 wherein each of said weightlines is made of a plurality of pipe sections filled with sand andjoined to each other by a rope.
 11. The combination as claimed in claim10, wherein said rope in one of said lateral weight lines is tied to oneof said drains.
 12. The combination as claimed in claim 2, wherein eachof said drains comprises a straight pipe and a flange affixed to saidstraight pipe along a central portion of said straight pipe, and to saidimpermeable membrane.
 13. The combination as claimed in claim 12,wherein said drain also comprises a sealed joint between said flange andsaid impermeable membrane.
 14. The combination as claimed in claim 12,wherein said straight pipe comprises an upper section extending abovesaid impermeable membrane and a lower section extending under saidflexible buoyant layer.
 15. The combination as claimed in claim 14,wherein said upper section has a length comprising an equilibriumdimension and a precautionary dimension, and said equilibrium dimensioncorresponds to a water level of said water ballast near one of saiddrain and said precautionary dimension extends above said water level ofsaid water ballast.
 16. The combination as claimed in claim 15, whereinsaid equilibrium dimension is between about one inch and about twoinches.
 17. The combination as claimed in claim 15, wherein saidprecautionary dimension is about one-half inch.
 18. The combination asclaimed in claim 15, wherein said water level of said water ballast ishigher than said wastewater level in said wastewater reservoir.
 19. Thecombination as claimed in claim 18, wherein said flexible buoyant layerhas a thickness and said water level of said water ballast is higherthan said wastewater level in said reservoir by a dimension which isslightly more than said thickness.
 20. The combination as claimed inclaim 12, wherein said straight pipe has a flap valve mounted therein.21. The combination as claimed in claim 12, wherein said straight pipehas a threaded portion on an upper end thereof.
 22. A method forinstalling a drainage system in a membrane cover covering a wastewaterreservoir, said method comprising the steps of: providing an array ofweight lines; disposing said array of weight lines on said membranecover; forming an array of troughs under said array of weight lines;providing a drain having a straight pipe; cutting a drain hole in saidmembrane cover under a weight line in said array of weight lines;allowing backwater from said reservoir to flow through said drain holeand into said array of troughs; submerging said array of weight lines insaid backwater until said backwater form a continuous pattern along saidarray of weight lines; measuring a depth of said backwater near saidweight line in said array of weight lines; adjusting said drain and saidstraight pipe in said drain hole with said straight pipe extending abovesaid membrane cover at least a dimension corresponding to said depth,and removing said backwater from said membrane cover. such that saidtroughs are usable for quickly accumulating rainwater therein forstabilizing said membrane cover during a rainstorm and said drains areusable for evacuating excess rainwater from said troughs.
 23. The methodas claimed in claim 22, wherein said step of providing an array ofweight lines comprises the step of providing said array of weight linesdefining a herringbone pattern.
 24. The method as claimed in claim 22,wherein said step of adjusting said drain and said straight pipe in saiddrain hole comprises the step of adjusting said drain and said straightpipe with said straight pipe extending above said membrane cover atleast a dimension corresponding to said depth plus a precautionarydimension of one-half inch.
 25. The method as claimed in claim 22,further comprising the step of providing said membrane cover having athickness of between about one-half inch to about three-quarter of aninch.